In order to print a color image, a copier or printer separates the image into primary ink colors, such as cyan, magenta, yellow, and black (CMYK). The copier or printer then prints layers of the primary inks separately one on top of the other to reproduce the full variety of colors in the image. This process is known as process color separation.
During process color separation, the printer or copier must accurately register the boundaries between adjacent colored areas. Unfortunately, accurate registration of color separation boundaries is difficult. Mis-registration of boundaries between colors often results in undesirable defects, such as gaps or overlaps, in the printed image.
In addition to color mis-registration, other types of errors can also occur during color printing. For example, “halo” effect errors can appear in a printed image as a side-effect of the xerographic marking process. As the term implies, “halo” effect is where a halo (or gap) appears at the interface between two colored regions. Mis-registration and halo are examples of a general class of errors described as “adjacency errors” which affect the boundary regions of adjacent colors.
For example, FIG. 1 illustrates an image 100 and examples of how a copier or printer may reproduce it. As shown, image 100 includes a first colored square area 104 that is surrounded by a second colored area 102. Image 106 is one example resulting from an error in the colored areas in image 100. This error may be due to color mis-registration, halo effect error, or a combination of both. As shown, image 106 may include a halo effect error, which can be seen as a space or gap 108. As another example, image 110 includes a gap 112 and a dark overlap area 114 that has been caused by a color mis-registration error of colored area 104 relative to colored area 102.
Trapping is one technique that may be used to compensate for color mis-registration. In general, trapping parameters are designed to overcompensate for an error because gaps between colored areas are considered highly undesirable. Thus, in conventional trapping, the colored areas 102 and 104 are intentionally grown or expanded into each other to fill in any gaps, such as gaps 108 or 112. For example, image 116 illustrates how trapping may be used to compensate for color mis-registration error shown in image 110.
Unfortunately, conventional trapping causes its own side effects, such as dark colored area 118, to appear in image 116. These dark colored areas are also undesirable. However, as long as these dark colored areas can be minimized, they are considered preferable over gaps (e.g., areas 108 or 112) at least in the case of these darker colors.
Conventionally, trapping parameters are determined for use across a group or fleet of copiers or printers. As a result, typical trapping parameters are “bidirectional” in nature to account for variations that can exist across a fleet of printers or copiers. That is, a conventional trapping parameter will specify growth of a colored area in both directions along an axis, such as both left and right (or +/−x) and both up and down (or +/−y). This allows a trapping parameter to correct for errors in virtually any situation and direction. However, this conventional approach to trapping tends to overcompensate for errors and fails to minimize the side effects of trapping, such as dark areas 114 and 118.
Therefore, it would be desirable to provide methods and apparatus that compensate for color printing errors, such as color mis-registration, halo effect error, or some combination thereof. It would also be desirable to provide methods and apparatus that minimize any side effects or artifacts of trapping.